1. Field of the Invention
The present invention relates to a gas purification capability measuring method for a gas purification apparatus of a getter system used in the manufacture of semiconductor devices and the like, and the gas purification apparatus.
2. Description of the Related Art
Gases used in the manufacture of semiconductor devices and the like must have higher purities than those used in general industrial applications. A gas purification apparatus is generally arranged in a gas line, and its gas purification capability is managed. In particular, for example, since the purity of argon (Ar) gas generally used as a line gas in a sputtering technique greatly influences reliability of resultant metal wiring layers, an Ar gas purification apparatus is generally used. According to a conventional method of monitoring the purification capability of this Ar gas purification apparatus, a sampling technique is used. In this sampling technique, purified Ar gas is sampled in a gas container. An analyzing means such as a gas chromatograph is used to detect a trace amount of an impurity. According to this sampling technique, however, a long period of time and high cost are required. For example, it takes a day to sample the gas and a week to analyze the sampled gas. It is difficult to periodically evaluate the gas. As a result, the gas purification apparatus is replaced with a new one in accordance with the due recommended by a gas purification apparatus manufacturer.
FIG. 1 is a schematic system diagram of a conventional gas purification apparatus and a vacuum apparatus associated therewith. More specifically, FIG. 1 shows a method of easily evaluating the purification capability of a gas purification apparatus for Ar gas or the like within a short period of time and an evaluation apparatus therefor so as to solve the conventional problem described above (Published Unexamined Japanese Patent Application No. 1-215343). The gas purification capability of the gas purification apparatus is defined as a capability of a gas purification unit for removing impurity gases such as N.sub.2 and O.sub.2 contained in a line gas such as Ar gas. The gas purification capability is represented by the ratio of a total amount of unremoved impurity gases to the amount of Ar gas at the outlet of the gas purification unit. The line gas such as Ar gas supplied from a container or the like through a main pipe 1 is supplied to a gas purification unit 2. The gas purification unit 2 incorporates a getter material, absorbs the impurity gases contained in the supplied Ar gas, and purifies the Ar gas. The purified Ar gas is supplied to a process chamber 3 in a sputtering apparatus. A valve 4 is arranged midway along the main pipe 1 connected to the inlet of the gas purification unit 2. A valve 5 and a flow controller 6 are arranged between the outlet of the gas purification unit 2 and the process chamber 3. A discharge pipe 7 of the process chamber 3 is connected to a vacuum pump 9 through a valve 8. An auxiliary piping system 10 is branched from the main pipe 1 at the outlet of the gas purification unit 2. An impurity gas supply tank 11, an auxiliary vacuum pump 12, a vacuum gauge 13, and valves 14, 15, and 16 are arranged midway along the auxiliary piping system 10. A vacuum gauge 17 is arranged at the outlet of the gas purification unit 2.
The gas purification capability of the gas purification unit 2 is monitored in accordance with the following procedures. First, the valves 4 and 5 of the main pipe 1 are closed, the valves 14 and 16 of the auxiliary piping system 10 are opened, and the valve 15 is closed. The auxiliary vacuum pump 12 is actuated to evacuate the gas purification unit 2, the impurity gas supply tank 11, and the auxiliary piping system 10, so that the internal pressure thereof is set to be about 1.times.10.sup.-1 to 1.times.10.sup.-3 Pa. The valves 14 and 16 are closed and the valve 15 is opened to supply an impurity gas such as N.sub.2 and O.sub.2 gases to the tank 11, and the pressure of the impurity gas inside the tank 11 is set to be about 100 to 1 Pa. The valve 15 is closed to seal the impurity gas in the tank 11, and then the valve 14 is opened. When the valve 14 is opened, the impurity gas in the tank 11 flows toward the gas purification unit 2 to temporarily increase the pressure at the outlet of the gas purification unit 2. In this case, the impurity gas is absorbed by the getter material, and the above internal pressure is immediately reduced. An absorb rate is obtained from the degree of decrease in this internal pressure, thereby monitoring the gas purification capability of the gas purification unit. This method can easily monitor the gas purification capability of the gas purification unit 2 within a short period of time as compared with the conventional method of sampling the Ar gas in a container and analyzing it.
According to the gas purification capability monitoring method by which the gas purification capability of a gas purification apparatus is represented by the concentration of the residual impurities contained in the purified gas at the outlet, it is difficult to detect degradation of the gas purification capability, i.e., the degradation of the getter material until the impurity concentration of the Ar gas at the outlet of the gas purification unit 2 reaches several hundreds of ppb since the pressure of the impurity gas is masked by the pressure of the line gas. In other words, it is difficult to detect degradation of the gas purification capability when the Ar gas has a high purity, an impurity of concentration of several hundreds of ppb or less. For this reason, the gas purification unit may be kept used while its gas purification capability is kept degraded. Since gases used in the manufacture of semiconductor devices are particularly required to have a higher purity in future applications, a simple method capable of monitoring a very low impurity concentration of about 10 ppb at the outlet of the gas purification unit must be required. As can be apparent from FIG. 1, since this gas purification apparatus has only one gas purification unit 2, it takes about 10 hours to cool the gas purification unit 2, evacuate the gas purification unit 2, and measure the impurity concentration so as to measure and evaluate the gas purification capability. During this period, gas supply to the gas purification unit must be interrupted. In this gas purification apparatus, a predetermined amount of an impurity gas must be supplied to the impurity gas supply tank 11 for every measurement of the gas purification capability. It therefore takes a long period of time to measure the impurity gas concentration. As a result, this method is not suitable for repeating the measurements.